The subject matter disclosed herein relates generally to a permanent magnet (PM) motor and, more specifically, to a PM motor having a physical construction such that the absolute position of the rotor may be determined without a separate position feedback device.
PM motors include a set of magnets in the rotor. The magnets are either inserted in slots within the rotor interior permanent magnets) or mounted to the outer surface of the rotor (i.e., surface permanent magnets). The magnets are configured such that the magnetic field varies in strength at different locations around the rotor. The magnets may be, for example, shaped such that more magnetic material exists in a central area of the magnet and less magnetic material exists toward the edge of the magnet or inserted within the rotor such that a portion of the magnet is further from the surface of the rotor. The magnetic field produced by the permanent magnets interacts with the field generated by a stator current to control rotation of the motor.
In order to control rotation of the PM motor, a motor drive generating the current for the stator windings must know the orientation of the rotor within the motor. Historically, it has been known to provide an external position feedback device, such as an encoder or resolver, along with the PM motor. Encoders and resolvers are coupled to the rotor and generate a signal corresponding to the angular position of the rotor. However, an external position feedback device adds increased space requirements and cost to the motor. Further, it is common that the environment in which the PM motor operates has electromagnetic noise. The conductors carrying the position signal between the feedback device and the motor drive must be shielded and/or filtered to reduce interference from the electromagnetic noise. The additional device also adds an additional point of failure within the control system.
In order to overcome some of these issues, efforts have been made to provide sensorless methods for determining the angular position of the rotor. Commonly, these sensorless methods inject a high frequency signal into the stator windings and monitor the resulting voltage and/or current signal responsive to the injected signal to identify an electrical angle of the rotor within one set of pole pairs. However, PM motors typically have multiple identical pole pairs and the electrical angle spans zero to three hundred sixty degrees over each pole pair. The motor drive, therefore, only knows the angular position of the rotor with respect to one electrical cycle and not an absolute mechanical position of the rotor. If, for example, a rotor has eight poles and, therefore, four pole pairs, the injected signal may determine that the rotor is at one of four different orientations within a complete revolution of the rotor. Thus, the injected signal may be used to determine a location of the rotor with respect to one set of pole pairs but it does not provide an absolute mechanical position of the rotor with respect to a full revolution of the rotor.
Thus, it would be desirable to provide an improved system for determining the absolute mechanical position of a rotor within a PM motor without utilizing an external position sensor.